Hydrocarbon oil generally contains sulfur compounds. When the hydrocarbon oil is used as a fuel, sulfur incorporated therein in the sulfur compounds is converted into sulfur oxides and is discharged into the atmosphere. Accordingly, it is preferred that such hydrocarbon oil have as low as possible a sulfur content from the viewpoint of avoiding air pollution upon combustion. This can be achieved by subjecting the hydrocarbon oil to a catalytic hydrodesulfurization process (HDP).
Since environmental pollution problems such as acid rain and nitrogen oxides (NO.sub.x) are of great concern world-wide, the removal of sulfur components from oil at the present technological level seems still insufficient. It is, in fact, possible to further reduce the sulfur content of hydrocarbon oil to some extent by operating the aforementioned HDP under more severe conditions, for example, by controlling the LHSV, temperature and pressure. HDP under such severe conditions, however, produces carbonaceous deposits on the surface of the catalyst, which, in turn, cause an abrupt drop in catalyst activity. The matter is even worse with a hydrocarbon oil which contains a light fraction, since HDP operated under severe conditions has a harmful influence, for example, on the color hue stability and storage stability of the oil. It can be seen that operational improvement is only effective to a certain extent, and more drastic measures are needed to develop a catalyst which is considerably increased in catalyst activity.
Hydrodesulfurization catalysts were conventionally produced by methods such as the so-called "impregnation process", which comprises impregnating a carrier with an aqueous solution having dissolved therein a salt of a metal belonging to Group VIII of the Periodic Table (sometimes referred to simply as a "Group VIII metal", hereinafter, and the same for one belonging to Group VIB of the Periodic Table) and a salt of a Group VIB metal, and, after drying, calcining the metal-impregnated carrier; the "coprecipitation process" which comprises adding an aqueous solution of a salt of a Group VIB metal and an aqueous solution of a salt of a Group VIII metal into an aqueous solution having dispersed therein alumina or a gel thereof to effect coprecipitation of a metal compound; and the "kneading process" which comprises kneading under heating a paste mixture composed of alumina or a gel thereof, an aqueous solution containing a salt of a Group VIB metal, and an aqueous solution containing a salt of a Group VIII metal, to remove water therefrom. For reference, see Ozaki, ed., Shokubai Chousei Kaqaku (Catalyst Preparation Chemistry), pp. 250 to 252, published by Kodansha Scientific.
None of the aforementioned methods, however, are suitable for uniformly dispersing a relatively large amount of metal compounds on the carrier. While it is possible to incorporate an excessive amount of the catalytically active metals in the carrier, there then arises another problem concerning the specific surface area of the catalyst. That is, increasing the amount of the catalytically active metals in the carrier reversely reduces the specific surface area of the catalyst which, as a result, inevitably sets a limit on the improvement in the desulfurizing activity of the catalyst. More specifically, despite that it has been reported that a carrier may carry a relatively large amount of an active metal, the practical content was confined to the range of from about 5 to about 8% by weight in the case of CoO, and from about 19 to about 20% by weight for MoO.sub.3.
With respect to a desulfurization process using a conventional catalyst, for example, catalytic hydrodesulfurization of a gas oil containing 1.3% by weight of sulfur carried out at a liquid hourly space velocity of 4 hr.sup.-1, at a reaction temperature of 350.degree. C., and under a reaction pressure of 35 kg/cm.sup.2, this process yields an oil where the sulfur content has been reduced to the range of from about 0.13 to about 0.19% by weight at best. In an another example, i.e., in the case of a vacuum gas oil (VGO) initially containing 2.50% by weight of sulfur, catalytic hydrodesulfurization at a liquid hourly space velocity of 0.4 hr.sup.-1, at a reaction temperature of 350.degree. C., and under a reaction pressure of 52 kg/cm.sup.2, yields a VGO oil the sulfur content of which is reduced only to an insufficient degree, with a limit being in the range of from about 0.15 to about 0.18% by weight. As a further example, a topped crude obtained from a crude oil with a 3.8% by weight sulfur content turns into a product where the sulfur content is lowered but the same is limited to the range of from about 0.9 to about 1.0% by weight, after catalytic hydrodesulfurization at a liquid hourly space velocity of 1.0 hr.sup.-1, at a reaction temperature of 361.degree. C., and under a reaction pressure of 150 kg/cm.sup.2.
The following is known: a process for preparing a catalyst obtained by calcining a heavy metal such as Ni and/or Mo, which has surface areas at least about 300 m.sup.2 /g, at least about 20% of the pore volume being in pores having diameters greater than about 350 .ANG. and at least about 20% of the pore volume being in pores having diameters less than about 70 .ANG. (U.S. Pat. No. 4,820,680); a catalyst (consisting essentially of a catalytic amount of at least one catalytic metal supported on gamma alumina) and its process obtained by a) hydrolyzing aluminium alkoxides to produce an aqueous alumina slurry and an organic reaction product, b) contacting said aqueous alumina slurry with an effective amount of a suitable organic solvent to form a solvent-aqueous alumina mixture, c) evaporating the solvent and water, and d) forming said alumina into particles and calcining to produce gamma alumina (U.S. Pat. No. 3,900,430). In these, however, the substantially obtained desulfurizing activity is at a similar level to those obtained in generally used catalysts. It is not desired to obtain significantly high desulfurizing activity.
Furthermore, since the physical properties (e.g., surface area, pore volume) and crystal structure of these catalysts have almost similar values to these of conventional catalysts by X-ray diffractometry, it can be understood that these catalysts have no substantial difference in structure.
It is desired to more readily obtain, without operating the HDP under severe operating conditions, a gas oil the sulfur content of which is reduced to the range of from about 0.05 to about 0.08% by weight, and, similarly, a VGO and a topped crude which are reduced in sulfur content to the range of from about 0.08 to about 0.10% and from about 0.6 to about 0.8%, respectively. If this would be possible, not only would the process become highly advantageous in economy from the viewpoint of prolonging the life of the catalyst, but also the resulting oil products would be effective in avoiding air pollution.
An object of the present invention is to develop a catalyst capable of containing a large amount of active metals yet which maintains a relatively high surface area, and which exhibits an extremely high desulfurization activity under ordinary operating conditions such that processing under severe conditions can be avoided.
Another object of the present invention is to provide a fuel oil from which the discharge of sulfur compounds at the time of combustion of the fuel oil is reduced to a level as low as possible, to thereby avoid air pollution.